Lim homeobox genes in the Ctenophore Mnemiopsis leidyi: the evolution of neural cell type specification

Using next generation sequencing, the first draft of the genome of the ctenophore Mnemiopsis leidyi has been generated. The Lhx genes in all animals are represented by seven subfamilies (Lhx1/5, Lhx3/4, Lmx, Islet, Lhx2/9, Lhx6/8, and LMO) of which four were found to be represented in the ctenophore lineage (Lhx1/5, Lhx3/4, Lmx, and Islet). Interestingly, the ctenophore Lhx gene complement is more similar to the sponge complement (sponges do not possess neurons) than to either the cnidarian-bilaterian or placozoan Lhx complements. Using whole mount in situ hybridization, the Lhx gene expression patterns were examined and found to be expressed around the blastopore and in cells that give rise to the apical organ and putative neural sensory cells.This research gives us a first look at neural cell type specification in the ctenophore M. leidyi. Within M. leidyi, Lhx genes are expressed in overlapping domains within proposed neural cellular and sensory cell territories. These data suggest that Lhx genes likely played a conserved role in the patterning of sensory cells in the ancestor of sponges and ctenophores, and may provide a link to the expression of Lhx orthologs in sponge larval photoreceptive cells. Lhx genes were later co-opted into patterning more diversified complements of neural and non-neural cell types in later evolving animals.LIM Homeobox (Lhx) genes were first isolated from the nematode, Caenorhabditis elegans, where the Lhx homolog, MEC-3, was shown to be required for the proper differentiation of touch receptor neurons [1]. Subsequent studies in C. elegans and rat isolated LIN-11 and Islet1, respectively, which together with MEC-3 are the founding members for the LIM family acronym [2,3]. Phylogenetically, Lhx genes were originally subdivided into six subfamilies, Lhx1/5, Lhx2/9, Lhx3/4, Lhx6/8, Islet, and Lmx [4]. Lhx proteins are composed of tandem zinc-finger LIM domains at the N-terminus, which function by binding specific co-factors that mediate t